1. Field of the Invention
The present invention relates to an apparatus and a method of driving a capacitive electromechanical transduction apparatus configured to receive an acoustic wave by a photoacoustic effect. In the present description, the term “acoustic wave” is used to describe a wide variety of acoustic waves including a sonic wave, an ultrasonic wave, and a photoacoustic wave, which are elastic waves generated (by the photoacoustic effect) in an object to be measured when the object to be measured is illuminated with light such as a near infrared ray (electromagnetic wave).
2. Description of the Related Art
An example of an electromechanical transduction apparatus for receiving/transmitting an ultrasonic wave is a CMUT (Capacitive Micromachined Ultrasonic Transducer) that is a capacitive ultrasonic transducer. The CMUT can be produced using a MEMS (Micro Electro Mechanical Systems) process based on a semiconductor process. It has been proposed to use a CMUT as an ultrasonic wave transducer (electromechanical transduction apparatus) in a measurement apparatus using a photoacoustic effect (see, for example, U.S. Patent Application Publication No. 2007/0287912).
To allow an object to be measured to generate an acoustic wave by the photoacoustic effect, for example, high-intensity light with a particular pulse width is periodically emitted by a light source and the object to be measured is illuminated with the emitted light. However, when the object to be measured is a living body, an acoustic wave is also generated by the photoacoustic effect at a skin on a surface of the living body. Furthermore, if another object located in the vicinity of the object to be measured is illuminated with the light, an acoustic wave is also generated by the photoacoustic effect. The acoustic waves generated in the above-described manner may reach an acoustic wave receiving unit. In this case, the acoustic waves received by the acoustic wave receiving unit do not include any information about the object to be measured, and thus the received acoustic waves function as noise. The noise of the acoustic wave originating from light emitted by the light source is greater in magnitude than the acoustic wave generated in the object to be measured (for example, by a light absorber such as a tumor existing in a living body). If such noise is input to the CMUT (serving as the electromechanical transduction apparatus) optimized for a particular magnitude of an acoustic wave generated by the object to be measured, the input noise can exert a great influence on the acoustic wave receiving operation of the CMUT. As described above, noise can cause a reduction in measurement accuracy in measuring an object to be measured.